The invention relates to a low-pressure mercury vapour discharge lamp having a satisfactory colour rendition, a colour temperature of the emitted white light of at least 2800 K. and a colour point (x.sub.L, y.sub.L) on or near the Planckian locus or curve, provided with a gas-tight radiation-envelope which contains mercury and a rare gas, and provided with a luminescent layer containing a luminescent halophosphate.
The expression "a satisfactory colour rendition" is to be understood to mean in the present description and the appended claims that the average colour rendering index Ra (average value of the rendering indices of eight test colours, as defined by the "Commission Internationale d'Eclairage"; Publication CIE No. 13.2 (TC-3.2), 1974) is at least 80.
The colour of visible radiation is characterized by the colour coordinates (x,y) which determine the colour point in the colour triangle (cf. publication CIE No. 15 (E-1.3.1), 1971). Lamps for general illumination purposes should emit light which can be considered to be "white". White radiation is found in the colour triangle at colour points located on the Planckian locus. This curve, also designated as curve of the black radiators and denoted hereinafter as the curve P, comprises the colour points of the radiation emitted by a completely black body at different temperatures (the so-called colour temperature). As the colour temperature of white radiation increases, the x-coordinate and, from a colour temperature of approximately 2500 K. also the y-coordinate, of the colour point have a smaller value. A given colour temperature is alotted not only to a given point on the curve P but also to radiation with colour coordinates located on a line intersecting the curve P at this point (see the said publication CIE No. 15). If this radiation has a colour point near the curve P, this radiation is also considered to be white light having this given colour temperature. In the present description and appended claims the expression "a colour point near the curve P" is to be understood to mean that the distance from the colour point to the point on the curve P with the same colour temperature is at most 20 MPCD. MPCD (Minimum Perceptible Colour Difference) is the unit of colour difference; see the publication of J. J. Rennilson in "Optical Spectra", October 1980, p. 63.
A large number of embodiments of low-pressure mercury vapour discharge lamps which have been known for may years and are still frequently used, comprise a luminescent material of the group of the alkaline earth metal halophosphates activated by Sb.sup.3+ and Mn.sup.2+. These lamps have the advantage that they are inexpensive and emit a satisfactorily high luminous flux. A great disadvantage of these lamps, however, is that their colour rendition properties leaves much to be desired. In general, they have Ra values of the order of 50 to 60 and only for lamps with a high colour temperature (for example, 5000 K.) is an Ra attained of approximately 75, which is not considered to be a satisfactory colour rendition.
For a long time, lamps have been known with which a satisfactory to very satisfactory colour rendition is obtained and which are provided with special luminescent materials. These lamps contain a tin-activated red-luminescing material on the basis of a strontium orthophosphate host, generally in combination with a blue-emitting halophosphate activated by Sb.sup.3+, in particular such as a strontium halophosphate. The said strontium orthophosphate luminesces in a very broad band which extends into the deep red. These known lamps have the disadvantages connected with the use of the said strontium-containing luminescent materials of exhibiting a comparatively small luminous flux and of a poor maintenance of luminous flux during life of the lamp. It has been found that the latter disadvantage renders it substantially impossible to use these materials in practice at a higher load by the radiation emitted by the mercury discharge.
It is true that high luminous fluxes and a satisfactory colour rendition can be obtained with lamps comprising three luminescent materials which emit in three comparatively narrow bands (cf. Dutch Patent Specification No. 164,697 (PHN. 7137)). Although these lamps have a high Ra value, certain colours are reproduced less satisfactorily due to the lack of red radiation having wavelengths above 620 nm. This becomes manifest in particular in a low value of R9 (rendering index for the deep red test colour No. 9).
If a high value of R9 is to be attained, a certain contribution in the red above 620 nm is necessary in the spectrum of the emitted radiation of a low-pressure mercury vapour discharge lamp. This is also the case with higher values of the colour temperature of the emitted radiation, although the required red contribution is larger as the colour temperature is lower. Inter alia for this reason the said tin-activated strontium orthophosphate was used in the aforementioned lamps having a satisfactory to very satisfactory colour rendition. For this material has an emission maximum at approximately 625 nm and a half value width of the emission band of approximately 150 nm so that the spectrum is filled satisfactorily also in the deep red.